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We have a 50 amp / 240 volt outlet in our garage for an electric car charger. We would like to use this power for snow melting our driveway in Massachusetts so as to avoid needing more power to be added to the house. We would never charge the car and snow melt at the same moment of time. I would have thought there would be some sort of manual switch that would allow me to select between "charge car" and "snow melt," but one electrician said that if we attach the snow melter, we could never charge the car. Each one is required to have its own dedicated circuit, and we would have to add power to the house. If this is the code? Why? Why can't I just be allowed to choose ?

To be clear, my existing setup is two 50amp breakers in a 100 amp subpanel dedicated for two electric cars.

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I gather I could theoretically have four 50 amp breakers where I now have two: two for the electric cars, and two for the snow melters. It would all be legal, if I got two generator interlock kits and each one pairs two of the 50 amp circuits, insuring that I could never use more than 100 amps at a time. Is that the idea?

Any reason why these interlocks couldn't be placed by the existing 100amp subpanel (shown here in the photo)?

  • Can you post a photo of the inside of the receptacle box please? – ThreePhaseEel Jan 18 at 4:44
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    While I think Harper has got a solution that works, if you are worried about the (one-time) cost of more service you might want to consider the running cost of electric snow melting, which is extreme and ongoing. – Ecnerwal Jan 18 at 14:45
  • What make/model is the existing subpanel? That is going to be key to making this work. – manassehkatz-Moving 2 Codidact Jan 19 at 15:03
  • I've never seen interlock kits that two would fit in a single panel. – NoSparksPlease Jan 19 at 16:26
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Straight up, this is a hard sell

First, EVSE's require a dedicated circuit. NEC 210.17.

Second, a 50A circuit can only serve one thing - NEC 210.23. So it cannot serve a hardwired driveway melter and also an EVSE receptacle. So that's right out.

Third, NEC 625.42 says the EVSE can be cord-and-plug connected if all are true:
1) it's part of a listed system
2) The receptacle <= 50A
3) It's cord connected to facilitate a) ready removal for interchange, b) maintenance and repair, or c) repositioning of the EVSE
4) Cord length <= 6 feet
5) Receptacles located to avoid physical damage to cord.

I think item 3 will be a hard sell.

So you're trying to argue that you should get an exception to the dedicated circuit rule (NEC 210.17); this outlet isn't an EVSE outlet -- it's a general purpose outlet that sometimes (wink, nudge) has a level 2 EVSE plugged into it. Hmmmm. That'll be an interesting sell to the AHJ. AHJ==your local inspector; the one that you pull the permit from.

The other prong of your argument, then, is that it's somehow reasonable for the driveway melter to be cord-and-plug-connected. In practice, that means it comes to a junction box, which then has a cord or inlet. This doesn't make a whole lot of sense, but it also absolutely requires the melter to call out a 50A breaker in its installation instructions. If the melter is looking for a 20A-40A breaker, no deal because the breakers don't match.

However, it would sell as a subpanel

If you convert this 50A circuit so it serves a subpanel instead of an EVSE, different deal. The subpanel does not need a "main breaker" if it's in the same building as it's served from.

In the subpanel you have a 50A breaker for the EVSE, and a ??A breaker for the driveway melter. That solves the breaker size problem.

Here's an interesting fact. Nothing in Code prohibits you from misusing a "generator interlock kit" to interlock any two random branch-circuit breakers, just for the chuckles. Only one can be on at a time. Go with either a Siemens/Murray panel or a Square D "QO". Use Siemens' ECSBPK02 interlock ($30), or a Square D's QO2DTI interlock ($25).

Now we open an ugly chapter: Provisioning. The feeder cable must be provisioned to be able to suppor--- Oh, wait. You've got an interlock there, don't you? Well, then. Nevermind.

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  • do you think this might pencil out better ($wise) with a subpanel and interlock, or a double throw switch? – batsplatsterson Jan 18 at 11:56
  • A 50 Amp double-throw switch? $$$ Plus the loss of "car charger is dedicated circuit" that having the sub-panel gives you, and having a switch won't... – Ecnerwal Jan 18 at 14:48
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    @batsplatsterson yeah, when switching these kinds of amperages, using service panel components as switches is pretty much beyond question Codewise, and will be recognized as legal. It's also often the cheapest way to do it because of economies of scale, same reason we use subpanels with main breakers when all we need is a disconnect switch. Thousands of times more panels are sold than large DPDT switches. – Harper - Reinstate Monica Jan 18 at 19:10
  • I don't think I have ever seen an interlock kit used this way but it's a smart way to go, clever - @ecnerwal - a 60A DT switch would be what $75 more in parts than the subpanel, interlock, and breakers - I don't think that is prohibitive. But granted it does get around the dedicated circuit requirement. – batsplatsterson Jan 19 at 11:16
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    @batsplatsterson plus, having read other forums,I am seeing where AHJs did not like the selector switch option, especially for EVSEs. They can't argue with the subpanel. – Harper - Reinstate Monica Jan 19 at 18:36

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